tools: mock some Python utils in v8.nix to reuse builds

tools/nix/v8.nix

@@ -33,13 +33,8 @@ let
         ../../deps/v8
         ../../node.gyp
         ../../node.gypi
-        ../../src/node_version.h
-        ../../tools/configure.d/nodedownload.py
-        ../../tools/getmoduleversion.py
-        ../../tools/getnapibuildversion.py
         ../../tools/gyp/pylib
         ../../tools/gyp_node.py
-        ../../tools/utils.py
         ../../tools/v8_gypfiles/abseil.gyp
         ../../tools/v8_gypfiles/features.gypi
         ../../tools/v8_gypfiles/ForEachFormat.py
@@ -103,6 +98,30 @@ stdenv.mkDerivation (finalAttrs: {
         patches+=("$filtered")
       fi
     done
+  ''
+  # We also need to mock some Python util files that are not used to configure Python.
+  # Mocking lets us avoid rebuilding the whole derivation if there's a unrelated
+  # change in one of those files.
+  + ''
+    mkdir -p tools/configure.d
+    cat -> tools/configure.d/nodedownload.py <<'EOF'
+    def parse(opt):
+      return {}
+    def help():
+      return ""
+    EOF
+    cat -> tools/getmoduleversion.py <<'EOF'
+    def get_version():
+      return "99"
+    EOF
+    cat -> tools/getnapibuildversion.py <<'EOF'
+    def get_napi_version():
+      return "9"
+    EOF
+    cat -> tools/utils.py <<'EOF'
+    def SearchFiles(dir, ext):
+      return []
+    EOF
   '';
   # We need to remove the node_inspector.gypi ref so GYP does not search for it.
   postPatch = ''